1. Field of the Invention
The present invention relates to a process for phosphating steel parts to improve corrosion and wear resistance and, more particularly, to a preliminary surface treatment of the parts.
2. Description of the Prior Art
The performances of parcels that have undergone phosphating, whatever their application, are known to depend essentially upon two characteristics: crystallinity and the quality of adherence of the phosphated layer to a substrate.
These two characteristics depend directly on the quality of the preparation of the metal surface prior to the phosphating operation properly speaking. The physical chemical state of this surface in fact conditions its reactivity in respect of the active agents of the phosphating bath, as well as the epitaxial growth of the layer of complex phosphates.
Generally speaking, it is sought, then, to achieve fine, regular crystallization, resulting in a layer as dense as possible.
Out of the whole of the layer building process, it is the germination stage which plays the predominant role in achieving this effect. That is why techniques have been sought after which are capable of permitting maximum activation of the metal surface and of creating thereon the largest possible number of germination sites. A certain number of solutions are known.
One of these solutions consists in electrolytically polishing the steel substrate, this being followed by depassivation and sanding with a cutting abrasive.
Another solution is to "seed" the surface with crystallization germs, for example, by pre-rinsing the untreated steel with a suspension of titanium salts.
Yet another solution is to adapt the composition of the phosphating bath, for example by using additives such as certain nickel or cadmium salts.
Finally, there are also known so-called "spraying" type techniques, wherein acicular crystallization is induced through the action of jets.
It is known, furthermore, that not all steels can be phosphated, particularly those containing over 5% of metallic addition elements, such as Cr, Mo and Ni, as layers with low coherence would be obtained in this case. This effect can be attenuated, to a certain extent, by specially adjusting the acidity of the phosphating bath, or by prior depassivation.
In conventional applications, and when the parts are not required to present exceptional qualities, the aforementioned techniques given industrialists satisfactory results, for example corrosion resistance permitting from 50 to 100 hours' exposure to a salt mist environment or even, for example, an improvement in running-in conditions during the very first hours thereof.
On the other hand, when corrosion and wear resistance requirement are more severe, these solutions prove inadequate. Other processes have thus been developed.
One example is provided in German Patent Application No. 28 53 542, which teaches that good protection against corrosion can be obtained for steel parts by initially nitriding the parts and then, in a second step, phosphating them, and, more particularly, phosphating them using manganese.
Variants of this technology are described in Japanese Patent Application No. 53-001647 and Soviet Union Patent Application No. 926070.
Here again, however, in view of the constantly increasing technical requirements with regard wear and corrosion resistance, the properties conferred on the parts by these processes also often prove inadequate nowadays. They cannot, for instance, guarantee the 400 hours' resistance to a salt mist environment desired by the automotive industry. In the case of gears made of carbonitrided steel subjected to severe stress, the surface phosphating is entirely removed in barely more than ten hours or so, that is to say in approximately the time required for running-in, with the resulting loss of the beneficial effect of the phosphated layer on the retention, and hence the stability, of lubricant films.